1. Background of the Invention
Advances in the performance of electrical and electronic equipment and the growing use of the same have caused an increase in electromagnetic interference in recent years. Displays such as CRTs and PDPs emit electromagnetic waves. A PDP is an assembly composed of a glass substrate having a data electrode and a fluorescent layer, and a glass substrate having a transparent electrode. When operated, such a display not only emits visible light that produces an image, but also generates electromagnetic waves, near infrared rays, and heat in large amounts.
In general, a front panel containing an electromagnetic wave shielding filter is mounted on the front of a PDP in order to shield electromagnetic waves. The required efficiency in shielding electromagnetic waves in a frequency band of 30 MHz to 1 GHz, emitted from the front of the display, is 30 dB or more.
Further, to make an image displayed on the display highly visible, it is required that the electromagnetic wave shielding filter be not highly visible (or that the invisibility of the electromagnetic wave shielding filter be high) even in a place bright with extraneous light (sunlight, light emitted from electric lights, etc.), and that the front panel, as a whole, has moderate transparency (visible light transmission, visible light transmittance).
Furthermore, there is a demand for a production process by which the electromagnetic wave shielding filter can be produced in a decreased number of steps and with high productivity.
2. Prior Art
The following conventional processes have been usually used to produce electromagnetic wave shielding filters having metal mesh layers.
A known process for producing an electromagnetic wave shielding filter is that an electrically conductive black ink containing a black colorant is applied to a transparent substrate by intaglio offset printing to form a mesh pattern, which is then plated with a metal (see Patent Documents 1 and 2, for example). However, the electromagnetic wave shielding filter obtained in this manner is disadvantageous in that, since the metal layer present on the side opposite to the transparent substrate and the metal layer present on the side faces of the mesh reflect extraneous light and glisten, a displayed image appears white and the invisibility of the mesh is low. If a mesh made only from an electrically conductive black ink is used to impart the function of shielding electromagnetic waves in order to prevent a displayed image from appearing white and also to ensure the invisibility of the mesh, the electromagnetic wave shielding filter is to have insufficient electromagnetic wave shielding ability. Moreover, in the production process, the plating step takes a longer time because the electrically conductive ink has high electrical resistance, and this leads to low productivity.
Another known process for producing an electromagnetic wave shielding filter is as follows: a PET film (transparent substrate) and a copper layer are laminated with an adhesive layer; by a photo-etching process, this copper layer is made into a mesh consisting of a large number of openings and line parts surrounding these openings; and all of the bare portions (both faces and the side faces) of the line parts of the copper layer are subjected to blackening treatment (see Patent Document 3, for example). An electromagnetic wave shielding filter obtained in this manner shows sufficiently high electromagnetic wave shielding ability. In addition, it has improved invisibility of the mesh and increased ability to prevent a displayed image from appearing white in the light, as compared with the electromagnetic wave shielding filters containing bare metal layers, disclosed in Patent Documents 1 and 2. However, chemical conversion treatment is conducted for the blackening treatment to produce needle crystals, so that before the copper layer covered with the blackening layer is laminated to a PET film, the blackening layer falls off or is deformed, and also readily undergoes change or decrease in blackness. Moreover, since high temperature treatment is conducted, the electromagnetic wave shielding filter tends to curl, and its external appearance thus becomes poor.
A further known process for producing an electromagnetic wave shielding filter is as follows: a hydrophilic resin layer containing a paradium catalyst is formed on a transparent substrate, and a metal comprising copper or nickel is deposited on this resin layer by electroless plating, thereby forming both a blackening layer and a metal layer on the back surface of the substrate; the blackening layer and the metal layer are then photolithographically made into a mesh; and another blackening layer, a black nickel layer, is formed on the face of the mesh and the side faces of the line parts of the mesh by electroplating to blacken the whole bare line parts. In this process, the blackening layer and the metal layer are continuously formed directly on the transparent substrate, so that the blackening layer does not easily fall off or deteriorate in the course of production. This process, however, is disadvantageous in that the metal layer and the blackening layer are easily separated from the transparent substrate because the strength and adhesive power of the hydrophilic resin is weak, and in that it takes a long time to form, by plating, the metal layer with a thickness (several tens of micrometers) great enough to shield electromagnetic waves.
Patent Document 1: Japanese Laid-Open Patent Publication No. 13088/2000,
Patent Document 2: Japanese Laid-Open Patent Publication No. 59079/2000,
Patent Document 3: Japanese Laid-Open Patent Publication No. 9484/2002, and
Patent Document 4: Japanese Laid-Open Patent Publication No. 77887/2000.